Download The Endocrine System

The Endocrine System
The endocrine system affects bodily activities by releasing chemical
messages, called hormones, into the bloodstream from exocrine and
endocrine glands. The function of hormones is to:

Control the internal environment by regulating its chemical
composition and volume

Respond to environmental changes to help the body cope with
emergencies - infection, stress etc

Help regulate organic metabolism and energy balance

Contribute to the management of growth and development
Hormones are chemicals that cause certain changes in particular parts of
the body. Their effects are slower and more general than nerve action.
They can control long-term changes such as rate of growth, rate of
activity and sexual maturity.
The endocrine or ductless glands secrete their hormones directly into the
blood stream. The hormones are circulated all over the body and reach
their target organ via the blood stream. When hormones pass through the
liver, they are converted by the kidneys. Tests on such hormonal products
in urine can be used to detect pregnancy.
The endocrine system consists of a series of glands that secrete
hormones; they are found throughout the body and include the pituitary,
thyroid, parathyroids, thymus, supra-renal or adrenal glands, part of the
pancreas and parts of the ovaries and testes. Although these glands are
separate, it is certain that they are functionally closely related because the
health of the body is dependent upon the correctly balanced output from
the various glands that form this system.
The Pituitary Gland (Hypophysis)
This gland has been described as the leader of the endocrine orchestra. It
consists of two lobes, anterior and posterior. The anterior lobe secretes
many hormones, including the growth-promoting somatotropic hormone
which controls the bones and muscles and in this way determines the
overall size of the individual. Over secretion of the hormone in children
produces gigantism and under secretion produces dwarfism. The anterior
lobe also produces gonadotropic hormones for both male and female
gonad activity. Thyrotropic hormones regulate the thyroid and
adrenocorticotropic hormones regulate the adrenal cortex. It also
produces metabolic hormones.
The posterior lobe produces two hormones - oxytocin and vasopressin.
Oxytocin causes the uterine muscles to contract; it also causes the ducts
of the mammary glands to contract and, in this way, helps to express the
milk that the gland has secreted into the ducts. Vasopressin is an
antidiuretic hormone that has a direct effect on the tubules of the kidneys
and increases the amount of fluid they absorb so that less urine is
excreted. It also contracts blood vessels in the heart and lungs and so
raises the blood pressure. It is not certain whether these two hormones are
actually manufactured in the posterior lobe or whether they are produced
in the hypothalamus and passed down the stalk of the pituitary gland to
be stored in the posterior lobe and liberated from there into the
circulation.
The Thyroid
The right and left lobes of this gland lie on either side of the trachea
united by the isthmus. Average size of each lobe is 4cm long and 2cm
across but these sizes may vary considerably. The secretion of this gland
is thyroxine and tri-iodothyronine. Thyroxine controls the general
metabolism. Both hormones contain iodine but thyronine is more active
than thyroxin. Under-secretion of this hormone in children produces
cretinism; the children show stunted growth (dwarfism) and fail to
develop mentally. Under secretion in adults results in a low metabolic
rate. Over secretion in adults gives rise to exophthalmic goitre and the
metabolic rate is higher than usual. Such persons may eat well but burn
up so much fuel that they remain thin. This is usually accompanied by a
rapid pulse rate. This gland, therefore, has a profound influence on both
mental and physical activity.
The Parathyroid Glands
There are four of these glands, two on either side lying behind the
thyroid. Their secretion is parathormone - the function of which is to raise
the blood calcium as well as maintain the balance of calcium and
phosphorus in both the blood and bone structures. Under secretion gives
rise to a condition known as tetany in which the muscles go into spasm,
and over secretion causes calcium to be lost to the blood from the bones
giving rise to softened bones, raised blood calcium and a marked
depression of the nervous system.
The Thymus Gland
This gland lies in the lower part of the neck and attains a maximum
length of about 6cm. After puberty, the thymus begins to atrophy so that
in the adult only fibrous remnants is found. Its secretion is thought to act
as a brake on the development of sex organs so that as the thymus
atrophies, the sex organs develop. Recent research into the activity of this
gland reveals that it plays an important part in the body's immune system
by producing T lymphocy - the T standing for thymus derived.
The Suprarenal or Adrenal Glands
These are two in number, triangular and yellow in colour. They lay one
over each kidney. They are divided like the kidney into two parts -the
cortex and the medulla. The cortex is the outer part of the gland and
produces a number of hormones called cortico-steroids. Their function is
to control sodium and potassium balance, stimulate the storage of glucose
and affect or supplement the production of sex hormones. The medulla or
inner layer produces adrenaline, a powerful vasoconstrictor. Adrenaline
raises vessels and raised the blood sugar by increasing the output of sugar
from the liver. The amount of adrenaline secreted is increased
considerably by excitement, fear, or anger, which has caused the adrenals
sometimes to be referred to as the glands of fright and fight.
The Gonads or Sex Glands
These glands are naturally different in men and women because they
serve different, though, in many respects, complementary functions. In
the female the gonads are the ovaries and in the male the testes. Female
sex hormones are oestrogen and progesterone. The male sex hormone is
testosterone, though each sex produces a small quantity of the opposite
hormone. The female hormones are responsible for developing the
rounded, feminine figure, breast growth, pubic and auxiliary hair and all
the normal manifestations of femininity and reproduction. Male hormone
is responsible for voice changes, increased muscle mass, development of
hair on the body and face and the usual development of manliness.
Pancreas
The endocrine part of the pancreas consists of clumps of cells called islets
of Langerhans that secrete insulin. Insulin regulates the sugar level in the
blood and the conversion of sugar into heat and energy. Too little insulin
results in a disease known as diabetes mellitus. This disease is divided
into one form, juvenile onset, which occurs before the age of 25, and
another form that begins in maturity. It is a very common disease. It is
known that some half million people in the United Kingdom suffer from
it sufficiently badly to need treatment but is has been estimated that there
are many more people in whom the disease exists at a sub-treatment
level.